The liquid crystal display device is classified based on the operation mode of liquid crystals, for example, into PC (phase change), TN (twisted nematic), STN (super twisted nematic), BTN (bistable twisted nematic), ECB (electrically controlled birefringence), OCB (optically compensated bend), IPS (in-plane switching), and VA (vertical alignment). The device is classified based on the driving system as PM (passive matrix) and AM (active matrix). PM (passive matrix) is classified into static, multiplex, etc. and AM is classified into TFT (thin film transistor), MIM (metal insulator metal), etc.
The liquid crystal display devices contain a liquid crystal composition having appropriate physical properties. In order to improve the characteristics of the device, it is preferred that the composition has appropriate physical properties. General properties necessary for a compound as a component of the composition include: (1) chemical stability and physical stability; (2) high clearing point (the clearing point is a transition temperature from a liquid crystal phase to an isotropic phase); (3) low lower limit temperature of a liquid crystal phase (the liquid crystal phase means a nematic phase, a smectic phase and the like); (4) low viscosity; (5) appropriate optical anisotropy; (6) appropriate dielectric anisotropy (compounds having high dielectric anisotropy often have high viscosity); and (7) high specific resistivity.
The composition is prepared by mixing various compounds. Accordingly, it is preferred that the compounds are well mixed with other compounds. Since the device is sometimes used at a temperature below a freezing point, compounds having favorable compatibility at a low temperature are preferred. Compounds having a high clearing point or a low lower limit temperature of a liquid crystal phase contribute to a wide temperature range of the nematic phase in the composition. A preferred composition has an optical anisotropy suitable to the low viscosity and the mode of the device. A high dielectric anisotropy of the compound contributes to a low threshold voltage of the composition. Such a composition can provide a device having such characteristics as a wide usable temperature range, a short response time, a large contrast ratio, a low driving voltage, a small power consumption, and a large voltage holding ratio.
The related art is disclosed in JP-A No. 2-228 (EP-A No. 315193, U.S. Pat. No. 5,059,345), EP-A No. 423520, JP-A No. 3-24036, JP-A No. 3-204835 (U.S. Pat. No. 5,128,062), JP-A No. 4-330040, and JP-A No. 5-294898. Further preferred liquid crystal compounds, liquid crystal compositions, and liquid crystal display devices are demanded.